The present invention relates generally to the production of chlorine by electrolysis. More specifically, the present invention relates to the conjoint production by an electrolytic cation exchange method of high purity elemental chlorine, high purity hydrogen, and alkali metal phosphates. Most specifically, the present invention relates to the conjoint synthesis of elemental chlorine and sodium or potassium phosphates from, respectively, sodium chloride or potassium chloride and phosphoric acid.
The world requirements for elemental chlorine grows at a very rapid rate. There is a similar but even larger increase in world food requirements and in the concomitant rate of fertilizer usage, especially in arid regions. The existing chlor-alkali processes based on sodium chloride are limited, on the one hand, by the need for the disposal at economic prices of large quantities of caustic and, on the other, by energy and environmental limitations. Where the more efficient mercury cathode cell is employed in the latter processes, there is the possibility of escape of mercury to the atmosphere and to streams and lakes. Also, brine purification and for most chlor-alkali processes and especially for the mercury cathode version is a continuous source of operating problems. A need exists, therefor, for a chlorine process which does not produce caustic, which involves minimal pollution risk, which is tolerant of brine and other raw material impurities, and which produces needed chlorine and phosphate salts with great efficiency.
For many years agricultural researchers have experimentally shown the great value of potassium phosphates as a single source of the potash and phosphorous so needed in fertilizers for good plant growth. Such phosphates are completely consumed leaving no residual ions to pollute the soil. Moreover, the potassium phosphates can be polymerized or condensed to have any desired water solubility and a range of potassium and phosphate release rates as well as offering a means of minimizing phosphate run-off to streams and lakes. The build-up of undesired acid ions in the soil, for example chloride ion from the "muriate of potash" (KCl) of the fertilizer industry, is especially bad in arid agriculture such as the irrigated farming of the Western United States and in the Middle East where sufficient water may not always be available to reduce such build-up by leaching.
However, after nearly a quarter century no proven, large scale commercial process is in operation producing large volumes of potassium phosphates at economic prices, particularly of the highly desirable KH.sub.2 PO.sub.4. Workers at the Tennessee Valley Authority reacted phosphorous pentoxide with potassium chloride and water at 1450.degree. to 1650.degree.F to produce potassium metaphosphate and by-product hydrogen chloride. The process suffered from the high costs incident to the use of P.sub.2 O.sub.5 and to the disposal of hydrogen chloride and, in addition, corrosion of equipment was reported as severe. Another known method not yet in large scale use involves reacting KCl and H.sub.2 SO.sub.4 producing KHSO.sub.4 carried in H.sub.2 SO.sub.4 and liberating hydrogen chloride followed by reacting the KHSO.sub.4 /H.sub.2 SO.sub.4 mixture with phosphate rock to produce KH.sub.2 PO.sub.4 in H.sub.3 PO.sub.4, and lastly by adding a precipitant such as methanol or acetone to precipitate the desired KH.sub.2 PO.sub.4. The latter process also requires an economical use to dispose of very large quantities of hydrogen chloride; it seems potentially subject to severe equipment corrosion; it appears to involve only partial conversion of the phosphoric acid; and it seems to involve several potentially difficult separation steps.
There exists a great need for a lower cost route to high purity chlorine and a low cost, practical method for producing sodium and potassium phosphates.